Image forming apparatus, belt driving device, and belt driving device control method

ABSTRACT

In an image forming apparatus, in which a belt driving device, provided in an image forming part, has: plural rollers including a driving roller; an endless belt; a driver, with one roller as a tension control roller to control tension applied to the endless belt, that moves the tension control roller in a vertical direction to an axis of another roller; a position detector that detects the position of the tension control roller; a pressure detector that detects pressure added with the endless belt to the tension control roller; and a controller that controls movement of the tension control roller. To set the tension applied to the endless belt as a predetermined value, the controller controls the driver to bring the detected pressure closer to a target pressure to be added to the tension control roller in the detected position of the tension control roller.

CROSS-REFERENCE TO RELATED APPLICATIONS

The entire disclosure of Japanese Patent Application No. 2017-157757,filed on Aug. 18, 2017, is incorporated herein by reference in itsentirety.

BACKGROUND Technological Field

The present invention relates to an image forming apparatus, a beltdriving device and a belt driving device control method.

Description of the Related Art

An image forming apparatus has a belt driving devices such as a conveyerbelt to convey a recording medium, and further an intermediate transferbelt to hold an image to be transferred to the recording medium. Thesebelt driving devices have a structure in which an endless belt isstretched around e.g. a driving roller, a driven roller, and a tensionadding roller. It is known that in the belt driving device having thisstructure, belt running becomes unstable due to expansion andcontraction of the belt accompanying temperature change. Accordingly, astructure to change tension added to the belt by moving the position ofthe driven roller or the tension adding roller in accordance with belttemperature detected with a temperature detector is proposed (see thefollowing Patent Literature 1).

CITATION LIST Patent Literature

[Patent Literature 1] Japanese Unexamined Patent Application PublicationNo. 2011-158614

SUMMARY

However, in the image forming apparatus, the performance of belt runningin the belt driving device has a significant impact on an ink imageformed on the recording medium or the precision of formation of an imageheld on the intermediate transfer belt. Accordingly, further improvementof the running performance of the belt is required in the belt drivingdevice.

The present invention has an object to provide an image formingapparatus capable of high-precision image forming by further improvingthe running performance of an endless belt, and further to provide abelt driving device and a belt driving device control method capable offurther improving the running performance of the endless belt.

To achieve at least one of the above objects, according to an aspect ofthe present invention, an image forming apparatus reflecting one aspectof the present invention is provided in which an image forming part isprovided with a belt driving device. The belt driving device includes: aplurality of rollers including a driving roller; an endless beltstretched around the plurality of rollers; a driver, with one of theplurality of rollers as a tension control roller to control tensionapplied to the endless belt, that moves a position of the tensioncontrol roller in a vertical direction to an axis of another roller; aposition detector that detects the position of the tension controlroller; a pressure detector that detects pressure applied with theendless belt to the tension control roller; and a controller thatcontrols movement of the tension control roller with the driver. To setthe tension applied to the endless belt as a predetermined value, thecontroller controls the driver to bring the pressure detected with thepressure detector closer to a target pressure to be added to the tensioncontrol roller in the position of the tension control roller detectedwith the position detector.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages and features provided by one or more embodiments of theinvention will become more fully understood from the detaileddescription given hereinbelow and the appended drawings which are givenby way of illustration only, and thus are not intended as a definitionof the limits of the present invention:

FIG. 1 is a main part block diagram of an image forming apparatusaccording to an embodiment;

FIG. 2 is a main part block diagram of a belt driving device provided inthe image forming apparatus according to the embodiment;

FIG. 3 is a functional block diagram of the belt driving deviceaccording to the embodiment;

FIG. 4 is a diagram explaining the relationship between a tensioncontrol roller position [x] and a tension control roller target pressure[F(x)];

FIG. 5 is a graph showing the relationship between the tension controlroller position [x] and the tension control roller target pressure[F(x)];

FIG. 6 is a flowchart showing a belt driving device control methodaccording to the embodiment;

FIG. 7 is a first diagram explaining the belt driving device controlmethod according to the embodiment; and

FIG. 8 is a second diagram explaining the belt driving device controlmethod according to the embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinbelow, an image forming apparatus, a belt driving device providedin the image forming apparatus, and a belt driving device controlmethod, to which the present invention is applied, will be described indue course. Note that in the following embodiment, an ink-jet imageforming apparatus will be given as the image forming apparatus, however,the image forming apparatus according to the present invention is widelyapplicable to structures having a belt driving device. Further, the beltdriving device to be described in the embodiment is not limited to thedevice provided in the image forming apparatus, but is widely applicableto belt driving devices required to have high running performance withrespect to an endless belt.

<<Image Forming Apparatus>>

FIG. 1 is a main part block diagram of the image forming apparatusaccording to an embodiment, showing an image forming part in the ink-jetimage forming apparatus viewed from a side direction. As shown in thisfigure, an ink-jet image forming apparatus 1 has a belt driving device 1a, an ink supply device 1 b, a heater 1 c, and a pressing roller 1 d.

In this configuration, the belt driving device 1 a is used forconveyance of a recording medium P in a predetermined direction. As therecording medium P, in addition to a normal paper sheet, a cardboard, acorrugated paper sheet, further, sheets formed of resin material, clothmaterial and the like, are widely used.

The belt driving device 1 a has a driving roller 10, a driven roller 11,a tension control roller 12, and an endless belt 13 stretched aroundthese rollers. The belt driving device 1 a rotates the endless belt 13by rotation of the driving roller 10. With this configuration, with anouter peripheral surface of the endless belt 13 as a placement surface13 s for the recording medium P, the recording medium P held on theplacement surface 13 s is conveyed in a rotating direction of theendless belt 13. This belt driving device 1 a has a driving mechanism 20characteristic of the present embodiment. The configurations of the beltdriving device 1 a and the driving mechanism 20 will be described indetail hereinafter.

The ink supply device 1 b is used for supplying ink to the recordingmedium P conveyed with the belt driving device 1 a. The ink supplydevice 1 b has a structure where ink heads to supply respective colorinks are provided along a conveyance direction of the recording medium Pconveyed with the belt driving device 1 a. As an example, aconfiguration where a yellow ink head 100 y, a magenta ink head 100 m, acyan ink head 100 c, and a black ink head 100 k are provided in orderalong the conveyance direction of the recording medium P is shown,however, the order of the ink heads is not limited to this order.

The heater 1 c is used for heating the recording medium P, supplied froman unshown medium supply part to the belt driving device 1 a, such thatthe recording medium P has a uniform temperature. Although detailedillustration is omitted here, the heater 1 c has a heating roller toheat the recording medium P supplied from the medium supply part andsend the recording medium P to the belt driving device 1 a side, and aheating chamber to uniform the temperature of the recording medium P,heated with the heating roller, in front of the belt driving device 1 a.Note that the heater 1 c is not limited to this structure as long as ithas a structure capable of heating and heat-retaining the recordingmedium P conveyed with the belt driving device 1 a. As the heater 1 chaving this structure, e.g., a structure to heat the endless belt 13 towhich the recording medium P is adsorbed, may be used.

The pressing roller 1 d is used for holding the recording medium P sentfrom the heater 1 c to the belt driving device 1 a between the placementsurface 13 s of the belt driving device 1 a and the pressing roller 1 d,to tightly press the recording medium P against the belt driving device1 a. This pressing roller 1 d may be any driven roller as long as it isprovided oppositely to the placement surface 13 s of the belt drivingdevice 1 a and rotated in accordance with movement of the recordingmedium P.

In the image forming apparatus 1 having the above-describedconfiguration, the recording medium P supplied from the medium supplypart is uniformly heated with the heater 1 c, and sent to the beltdriving device 1 a. Further, the recording medium P sent to the beltdriving device 1 a, in a status where it is tightly pressed with thepressing roller 1 d against the placement surface 13 s of the beltdriving device 1 a, is conveyed in a predetermined direction. Then therespective color inks are sequentially supplied from the ink supplydevice 1 b with respect to the recording medium P, in a status where itis conveyed with the belt driving device 1 a in the predetermineddirection, thus an image is formed on the recording medium P.

In this image forming apparatus 1, the endless belt 13 of the beltdriving device 1 a is expanded longer by contact with the recordingmedium P heated to a predetermined temperature. When the endless belt 13is expanded longer, a skid or meandering easily occurs in running of theendless belt 13. To maintain the precision of the image formed on therecording medium P, it is necessary to maintain good running performanceof the endless belt 13 without influence of expansion and contractiondue to heating, thus to improve conveyance of the recording medium Pwith the belt driving device 1 a. In this manner, the belt drivingdevice 1 a for conveyance of the recording medium P has a configurationas follows.

<<Belt Driving Device 1 a>>

The belt driving device 1 a has, in addition to the above-describeddriving roller 10, the driven roller 11, the tension control roller 12,and the endless belt 13, a supporting body 14 to support the endlessbelt 13 from the inner peripheral side, and a suction fan 15.

<Driving Roller 10>

In this configuration, the driving roller 10 is a roller which has anunshown driving motor and which freely rotates in a predetermineddirection. The driving roller 10 is provided with a rotational speedmeasuring unit such as a rotary encoder. The rotational speed measuredwith the measuring unit is fed back to the driving motor, thus therotational speed is freely controlled.

<Driven Roller 11>

The driven roller 11 is a roller provided in parallel to the drivingroller 10 with an interval where the ink supply device 1 b is provided,between the driving roller 10 and the driven roller 11. Further, thedriven roller 11 holds the recording medium P between the pressingroller 1 d and the driven roller 11. The driven roller 11 is rotatablein accordance with the rotating operation of the endless belt 13 by therotation of the driving roller 10.

<Tension Control Roller 12>

The tension control roller 12 is a roller provided in approximatelyparallel to the driving roller 10 and the driven roller 11, between thedriving roller 10 and the driven roller 11. The tension control roller12 adds pressure from the inner peripheral side toward the outerperipheral side, to the endless belt 13. The tension control roller 12is a driven roller, and also has a function as a steering roller tocontrol meandering of the endless belt 13. This tension control roller12 is provided with the same distance from the driving roller 10 andfrom the driven roller 11, as an example, however, the tension controlroller 12 is not limited to this arrangement. Further, this tensioncontrol roller 12 is provided with the driving mechanism 20 to controlthe pressure added to the endless belt 13. The details of the drivingmechanism 20 will be described hereinafter.

<Endless Belt 13>

The endless belt 13 is stretched around the driving roller 10, thedriven roller 11, and the tension control roller 12. The endless belt 13has plural through holes enabling passage of air between the outerperipheral surface as the placement surface 13 s for the recordingmedium P and an inner peripheral surface. These through holes areprovided in correspondence with a position where the recording medium Pis placed in a width direction of the endless belt 13.

Further, this endless belt 13 is made of e.g. steel, however, it is notlimited to this material. The endless belt 13 is expanded and contractedby temperature change. Accordingly, in the endless belt 13 which conveysthe recording medium P heated to the predetermined temperature, uponcontinuous conveyance processing of the recording medium P, especiallythe length in the rotating direction is expanded longer when heated bycontact with the recording medium P.

<Supporting Body 14>

The supporting body 14 is a plate member to support the endless belt 13,between the driving roller 10 and the driven roller 11, from the innerperipheral side. The supporting body 14 has plural through holescommunicating with the through holes provided in the rotating endlessbelt 13, to enable passage of air. This supporting body 14 may be formedof porous material.

<Suction Fan 15>

The suction fan 15 is provided between the driving roller 10 and thedriven roller 11 on the inner peripheral side of the endless belt 13 viathe supporting body 14. The suction fan 15 sucks the air on theplacement surface 13 s side in the endless belt 13 via the through holesof the supporting body 14 and the through holes of the endless belt 13.The belt driving device 1 a causes the recording medium P, supplied onthe placement surface 13 s of the endless belt 13, to be sucked to theplacement surface 13 s with the driving of this suction fan 15, betweenthe driving roller 10 and the driven roller 11, and conveys therecording medium P in the sucked state in the rotating direction of theendless belt 13.

<Driving Mechanism 20 of Tension Control Roller 12>

FIG. 2 is a main part block diagram of the belt driving device 1 aprovided in the image forming apparatus according to the embodiment,showing the main parts of the belt driving device 1 a shown in FIG. 1viewed from a front direction. Further, FIG. 3 is a functional blockdiagram of the belt driving device according to the embodiment.Hereinbelow, the details of the driving mechanism 20 of the tensioncontrol roller 12 will be described based on FIG. 2 and FIG. 3, andfurther, referring to FIG. 1.

As shown in these figures, the tension control roller 12 has, as itsdriving mechanism 20, a driver 21, a position detector 22, a pressuredetector 23, and a controller 24. The driver 21, the position detector22, and the pressure detector 23 are respectively provided at both endsof the tension control roller 12. With this configuration, the tensioncontrol roller 12 controls both end positions individually, and alsofunctions as a steering roller to adjust the position of the endlessbelt 13. Hereinbelow, the detailed configurations of the driver 21, theposition detector 22, the pressure detector 23, and the controller 24,provided in the driving mechanism 20, will be described.

[Driver 21]

The driver 21 has two support members 21 a to rotatably support the bothends of the tension control roller 12, shafts 21 b respectively extendedfrom the two support members 21 a, and actuators 21 c to move therespective shafts 21 b in extending directions. The two shafts 21 b areextended vertically to an axial direction of the driving roller 10 andthe driven roller 11. With this configuration, the both ends of thetension control roller 12 move a position [x], by driving of the twoactuators 21 c, in the vertical direction to the axial direction of thedriving roller 10 and the driven roller 11. Note that the position [x]is the position [x] at respective both ends of the tension controlroller 12. Hereinbelow, for the sake of simplification of explanation,the position will also be simply referred to as “position [x]” of thetension control roller 12.

[Position Detector 22]

The position detector 22 is used for detection of the position [x] ofthe tension control roller 12 moved with the driver 21. The positiondetector 22 may be provided in actuator controllers of e.g. theactuators 21 c of the two drivers 21. The position detector 22 detectsthe position [x] at the end of the tension control roller 12 based ondriving position of the actuator 21 c, and continuously detects theposition [x].

[Pressure Detector 23]

The pressure detector 23 is a load cell to detect pressure [f] added tothe tension control roller 12 with the endless belt 13 stretched aroundthe tension control roller 12. The pressure [f] detected with thepressure detector 23 is pressure toward the inner peripheral side of theendless belt 13 in the extending direction of the shaft 21 b of thedriver 21. Further, when the length of the endless belt 13 is constant,the pressure [f] detected here depends on the position [x] of thetension control roller 12. This pressure detector 23 is attachedrespectively to the shafts 21 b of the two drivers 21, to respectivelydetect the pressure [f] added to the ends of the tension control roller12. Further, these pressure detectors 23 continuously detect thepressure [f].

[Controller 24]

The controller 24 is connected to the two drivers 21, the positiondetectors 22, and the pressure detectors 23. The controller 24 controlsdriving of the drivers 21 based on the values detected with the positiondetectors 22 and the pressure detectors 23, such that a predeterminedtension [N] is applied to the endless belt 13 stretched around thetension control roller 12. Note that the predetermined tension [N]applied to the endless belt 13 is tension for rotation of the endlessbelt 13 with good runnability without skid or meandering. The details ofthe control of the driver 21 with the controller 24 will be described inthe following belt driving device control method. Further, thecontroller 24 has an input/output controller 24 a, a memory 24 b, and atarget pressure calculator 24 c as follows.

—Input/Output Controller 24 a—

The input/output controller 24 a is connected to the above-describeddriver 21 (more specifically, the actuator 21 c of the driver 21), theposition detector 22, and the pressure detector 23, further, connectedto the memory 24 b and the target pressure calculator 24 c.

The input/output controller 24 a causes the target pressure calculator24 c to calculate a target pressure [F(x)] in the position [x] based onthe position [x] of the tension control roller 12 detected with theposition detector 22 and data stored in the memory 24 b. Note that thetarget pressure [F(x)] is pressure to be added to the tension controlroller 12 in the position [x] to apply the predetermined tension [N] tothe endless belt 13, the length of which has been changed due totemperature change or the like, and is a value linked to the position[x].

Further, the input/output controller 24 a causes driving of theactuators 21 c based on the target pressure [F(x)] calculated with thetarget pressure calculator 24 c and the pressure [f] detected with thepressure detector 23. Thus the input/output controller 24 a controlsdriving of the belt driving device 1 a.

This input/output controller 24 a is a computer formed with a CPU, aROM, and a RAM, and performs the method of controlling the belt drivingdevice 1 a described hereinbelow by executing a program recorded on theROM or RAM with the CPU.

—Memory 24 b—

The memory 24 b is a ROM or a RAM to hold the target pressure [F(x)]linked to the position [x] of the tension control roller 12. The memory24 b holds, as the target pressure [F(x)] linked to the position [x] ofthe tension control roller 12, relational expression between theposition [x] of the tension control roller 12 and the target pressure[F(x)], or holds a table showing the linkage between the position [x] ofthe tension control roller 12 and the target pressure [F(x)].

FIG. 4 is a diagram explaining the relationship between the position [x]of the tension control roller 12 and the target pressure [F(x)] of thetension control roller 12. As shown in this figure, when the endlessbelt 13 is expanded longer by e.g. heating, to maintain thepredetermined tension [N] as a tension applied to the endless belt 13,the position [x] of the tension control roller 12 may be moved in adirection away from the driving roller 10 and the driven roller 11 by anappropriate amount.

Note that the position [x] of the tension control roller 12 is freelymoved in the vertical direction to the axial direction of the drivingroller 10 and the driven roller 11. Accordingly, when the endless belt13 is expanded longer by heating, the position [x] of the tensioncontrol roller 12 is moved from the position [x]=x0 (=0) as a referenceposition in the vertical direction to the axial direction of the drivingroller 10 and the driven roller 11 by an appropriated value, to aposition [x]=x1. Thus it is possible to maintain the predeterminedtension [N] as the tension applied to the endless belt 13.

Further, when the position [x] of the tension control roller 12 ischanged in this manner and the predetermined tension [N] is applied tothe endless belt 13, the value of the pressure [f] added to the tensioncontrol roller 12 is increased in accordance with expansion of theendless belt 13 and increase of the position [x]. That is, the pressure[f] added to the tension control roller 12 is a value represented with afunction having the position [x] as a variable. Accordingly, in theposition [x] of the tension control roller 12, to apply thepredetermined tension [N] to the endless belt 13 having a certainlength, the pressure to be applied to the tension control roller 12 isthe target pressure [F(x)] as a value linked to the position [x].

FIG. 5 is a graph showing a relational expression (1) between theposition [x] of the tension control roller and the target pressure[F(x)] of the tension control roller to apply the predetermined tension[N] to the endless belt 13 in respective lengths. As shown in therelational expression (1), when the endless belt 13 is made of steel,the position [x] and the target pressure [F(x)] are in relationshiprepresented by a linear function. The relational expression differs inaccordance with design of the belt driving device 1 a including materialand the size of the endless belt 13. Accordingly, it is previouslyobtained by preliminary experiment or the like and is stored in thememory 24 b.

—Target Pressure Calculator 24 c—

The target pressure calculator 24 c calculates the target pressure[F(x)] to set the tension [N] applied to the endless belt 13 in theposition [x] as a predetermined value, from the position [x] of thetension control roller 12 detected with the position detector 22 anddata stored in the memory 24 b. The data stored in the memory 24 b isthe target pressure [F(x)] linked to the above-described position [x] ofthe tension control roller 12. The data may be the above-describedrelational expression, or may be the table. When the data stored in thememory 24 b is a table, the target pressure calculator 24 c compares theposition [x] of the tension control roller 12 detected with the positiondetector 22 with the table stored in the memory 24 b, and selects atarget pressure [F(x)].

<<Belt Driving Device Control Method>>

FIG. 6 is a flowchart showing a belt driving device control methodaccording to the embodiment. The belt driving device control methodshown in the flowchart is the method of controlling the belt drivingdevice 1 a performed with the controller 24 of the above-describeddriving mechanism 20. Hereinbelow, referring to the functional blockdiagram of FIG. 3 and the graph of FIG. 5, the procedure of the methodof controlling the belt driving device 1 a will be described along withthe flowchart of FIG. 6. Note that the processing shown in the flowchartof FIG. 6 is started when the power source of e.g. the belt drivingdevice 1 a or the power source of an image forming apparatus having thebelt driving device 1 a is turned on, as a trigger.

[Step S101]

At step S101, the input/output controller 24 a obtains the position [x]of the tension control roller 12 detected with the position detector 22.As the position [x] obtained immediately after the start of theprocessing is, e.g., an initial position [x]=x0 (=0).

[Step S102]

At step S102, the input/output controller 24 a calculates the targetpressure [F(x)] from the relational expression (1) stored in the memory24 b and the position [x] obtained at step S101, by the control of thetarget pressure calculator 24 c. The target pressure [F(x)] is pressureto be added to the tension control roller 12 in the position [x] so asto apply the predetermined tension [N] to the endless belt 13. Forexample, the target pressure [F(x0)] corresponding to the initialposition [x]=x0 immediately after the start of the processing iscalculated here.

[Step S103]

Next, at step S103, the input/output controller 24 a updates the storedtarget pressure [F(x)] to the target pressure [F(x)] calculated at stepS102 (e.g. [F(x0)]).

[Step S104]

At step S104, the input/output controller 24 a obtains the pressure [f]of the tension control roller 12 detected with the pressure detector 23.As the pressure [f] obtained immediately after the start of theprocessing, e.g., pressure [f]=f0 holds.

[Step S105]

At step S105, the input/output controller 24 a compares the pressure [f]obtained at step S104 with the target pressure [F(x)] updated at stepS103, to determine whether or not a difference [d] between the obtainedpressure [f](e.g. pressure [f]=f0) and the target pressure [F(x)] (e.g.[F(x0)] is within an error range. Note that the error is a previouslyset value as a range not to influence the running of the endless belt13.

When the input/output controller 24 a determines that the difference [d]between the obtained pressure [f] and the target pressure [F(x)] iswithin the error range (YES), it proceeds to step S106. On the otherhand, when the input/output controller 24 a determines that thedifference is not within the error range (NO), it proceeds to step S106a.

[Step S106]

At step S106, the input/output controller 24 a determines whether or notpressurization/decompression processing is in progress at present, basedon the control state of the driver 21. The “pressurization/decompressionprocessing is in progress” means that processing to change the pressureadded from the endless belt 13 to the tension control roller 12 bychange of the position [x] of the tension control roller 12 with thedriver 21 is being performed. When the input/output controller 24 adetermines that the pressurization/decompression processing is inprogress (YES), it proceeds to step S107. On the other hand, when theinput/output controller 24 a determines that thepressurization/decompression processing is not in progress (NO), itreturns to step S101.

[Step S107]

At step S107, the input/output controller 24 a stops thepressurization/decompression processing by the control of the driver 21.In this case, it may stop the position [x] of the tension control roller12 with the driver 21. Thereafter, it returns to step S101.

[Step 106 a]

On the other hand, at step S106 a, the input/output controller 24 adetermines whether or not the pressure [f] obtained at step S104 isequal to or lower than the target pressure [F(x)] updated at step S103.Then, when the input/output controller 24 a determines that the pressure[f] is equal to or lower than the target pressure [F(x)] (YES), itproceeds to step S107 a. On the other hand, when the input/outputcontroller 24 a determines that the pressure [f] is not equal to orlower than the target pressure [F(x)] (NO), it proceeds to step S107 b.

FIG. 7 is a first diagram explaining the belt driving device controlmethod according to the embodiment. As shown in this figure, at stepS106 a, when it is determined that the pressure [f] is equal to or lowerthan the target pressure [F(x)] (YES), the obtained pressure [f] (=f0)is lower than the target pressure [F(x)] (=[F(x0)]). Further, itsdifference [d] is greater than the previously set error range.

[Step S107 a]

Then at step S107 a, the input/output controller 24 a startspressurization processing by the control of the driver 21. Thepressurization processing is processing to move the position [x] of thetension control roller 12 in the direction away from the driving roller10 and the driven roller 11, so as to increase the pressure added withthe endless belt 13 to the tension control roller 12, and is processingto bring the pressure [f] closer to the target pressure [F(x)].

At this time, when the pressurization processing is already performed,the pressurization processing is continued. Further, when decompressionprocessing is performed, the decompression processing is stopped and thepressurization processing is started.

The speed of the pressurization processing, i.e. the moving speed of theposition [x] of the tension control roller 12 is set such that thedifference [d] between the target pressure [F(x)] updated at step S103and the pressure [f] obtained at step S104 becomes within the errorrange as soon as possible. Further, after the end of this startingprocessing, the process returns to step S101.

[Step S107 b]

On the other hand, at step S107 b, the input/output controller 24 astarts decompression processing by the control of the driver 21. Thedecompression processing is processing to move the position [x] of thetension control roller 12 in a direction approaching the driving roller10 and the driven roller 11, so as to reduce the pressure added with theendless belt 13 to the tension control roller 12, and is processing tobring the pressure [f] closer to the target pressure [F(x)].

At this time, when the decompression processing is already performed,the decompression processing is continued. Further, when thepressurization processing is performed, the pressurization processing isstopped, and the decompression processing is started.

The speed of this decompression processing, i.e., the moving speed ofthe position [x] of the tension control roller 12 is set such that thedifference between the target pressure [F(x)] updated at step S103 andthe pressure [f] obtained at step S104 becomes within the error range assoon as possible. Further, after the end of this start processing, theprocess returns to step S101.

FIG. 8 is a second diagram explaining the belt driving device controlmethod according to the embodiment. As shown in this figure and FIG. 5,in the repeated processing at and after step S101, in correspondencewith the position [x]=x1 obtained at the next step S101, a new targetpressure [F(x1)] is calculated at the next step S102. Further, at stepS104, a new pressure [f]=f1 is obtained. As shown in the figure, thenewly calculated target pressure [F(x1)] may be lower, or may be higher,than the new pressure [f]=f1. Then the pressurization processing or thedecompression processing by the control of the driver 21 is continueduntil it is determined at step S105 that the difference[d] between thesepressures is within the error range, so as to converge the tensionapplied to the endless belt 13 to a predetermined tension [F(x)].

Effects of the Embodiment

The above-described belt driving device 1 a according to the embodimenthas a configuration that the pressure [f] added to the tension controlroller 12 is converged to the target pressure [F(x)] to apply thepredetermined tension [N] to the endless belt 13 with the position [x]of the tension control roller 12, and by feed-back of the pressure [f]added from the endless belt 13 to tension control roller 12.Accordingly, it is possible to effectively converge the tension of theendless belt 13 to the predetermined tension [N] in correspondence withexpansion and contraction of the endless belt 13, to further improve therunning performance of the endless belt 13. With this configuration, itis possible to infallibly prevent skid or meandering of the endless belt13 expanded longer by heating.

Note that the smaller the angle with the tension control roller 12 as avertex in the extending direction of the endless belt 13 is, the greaterthe change of the target pressure [F(x)] corresponding to the change ofthe position [x] of the tension control roller 12 is. Accordingly, bydownsizing of the device structure, it is possible to notably obtain theeffects of the above-described embodiment in accordance with reductionof the angle with tension control roller 12 as a vertex in the extendingdirection of the endless belt 13. Further, it is possible to control therunnability of the endless belt 13 with higher precision.

Further, by the above-described feed-back, the position [x] of thetension control roller 12 is changed as needed by moving the tensioncontrol roller 12 at a predetermined speed. Accordingly, it is possibleto quickly converge the pressure [f] added to the tension control roller12 to the target pressure [F(x)]. With this configuration, it ispossible to stabilize the runnability of the endless belt 13 at a highspeed.

Further, by individually moving the both ends of the tension controlroller 12, it is possible to control the runnability of the endless belt13 with higher precision.

Further, according to the image forming apparatus 1 having theabove-described belt driving device 1 a, it is possible to improve imageforming precision.

<<Modifications>>

In the above-described embodiment, one relational expression (or table)is stored in the memory 24 b. However, it may be configured such thatplural relational expressions (or tables) are stored in the memory 24 b,the input/output controller 24 a selects an appropriate relationalexpression from the plural relational expressions in accordance with usestatus of the endless belt 13, and uses the selected relationalexpression in the calculation of the target pressure [F(x)] at stepS102. Note that the plural relational expressions (or tables) arepreviously prepared.

As an example, in a case where the belt driving device 1 a is used inthe image forming apparatus 1, the plural relational expressions (ortables) are prepared in correspondence with the number of images formedwith the image forming apparatus 1. With this configuration, when apaper sheet is used as the recording medium P, it is possible to controlthe runnability of the endless belt 13 considering that the endless belt13 becomes slippery due attachment of paper powder to the endless belt13.

Further, in the above-described embodiment, the position [x] of thetension control roller 12 is moved at a predetermined speed upon startof the pressurization processing at step S107 a and upon start of thedecompression processing at step S107 b. However, it may be configuredsuch that at these steps, the position [x] of the tension control roller12 is moved by a predetermined amount. In this case, step S106 and stepS107 are omitted. When it is determined as (YES) at step S105, theprocess returns to step S101.

Further, in the above-described embodiment, the roller provided inapproximately parallel to the driving roller 10 and the driven roller11, between the driving roller 10 and the driven roller 11, is thetension control roller 12. However, it may be configured such that theroller provided in approximately parallel to the driving roller 10 andthe driven roller 11, between the driving roller 10 and the drivenroller 11, is merely a driven roller or a steering roller, and thedriving mechanism 20 is provided in the shown driven roller 11, as atension control roller.

Further, in the above-described embodiment, the image forming apparatus1 is an ink-jet type apparatus, however, the present invention is notlimited to this type. For example, the invention is also applicable toan electrophotographic type image forming apparatus using toner. In thiscase, the belt driving device 1 a is used as an intermediate transferbelt to hold a toner image, and a developing device is providedoppositely to the outer peripheral surface of the endless belt 13.Further, the outer peripheral surface of the endless belt 13 is used asa toner image holding surface to hold a toner image formed with tonersupplied with the developing device. In this case, it is also possibleto obtain a similar effect to that obtained in the above-describedembodiment.

Although embodiments of the present invention have been described andillustrated in detail, the disclosed embodiments are made for purposesof illustration and example only and not limitation. The scope of thepresent invention should be interpreted by terms of the appended claims.

REFERENCE SIGNS LIST

-   -   1 . . . image forming apparatus (image forming part)    -   1 a . . . belt driving device    -   1 b . . . ink supply device    -   10 . . . driving roller    -   11 . . . driven roller    -   12 . . . tension control roller    -   13 . . . endless belt    -   21 . . . driver    -   22 . . . position detector    -   23 . . . pressure detector    -   24 . . . controller    -   24 b . . . memory    -   24 c . . . target pressure calculator    -   P . . . recording medium    -   [N] . . . predetermined tension    -   [f] . . . pressure added to tension control roller    -   [F(x)] . . . target pressure    -   [x] . . . tension control roller position

What is claimed is:
 1. An image forming apparatus in which an imageforming part is provided with a belt driving device, wherein the beltdriving device including: a plurality of rollers including a drivingroller; an endless belt stretched around the plurality of rollers; adriver, with one of the plurality of rollers as a tension control rollerto control tension applied to the endless belt, that moves a position ofthe tension control roller in a vertical direction to an axis of anotherroller; a position detector that detects the position of the tensioncontrol roller; a pressure detector that detects pressure applied withthe endless belt to the tension control roller; and a controller thatcontrols movement of the tension control roller with the driver,wherein, to set the tension applied to the endless belt as apredetermined value, the controller controls the driver to bring thepressure detected with the pressure detector closer to a target pressureto be added to the tension control roller in the position of the tensioncontrol roller detected with the position detector.
 2. The image formingapparatus according to claim 1, wherein the controller has a memory tohold the target pressure linked to the position of the tension controlroller.
 3. The image forming apparatus according to claim 1, wherein thecontroller has a target pressure calculator that calculates the targetpressure based on the position of the tension control roller detectedwith the position detector; and controls the driver so as to reduce adifference between the calculated target pressure and the pressuredetected with the pressure detector.
 4. The image forming apparatusaccording to claim 1, wherein the controller moves the tension controlroller with the driver at a predetermined speed, and when the differencebetween the target pressure and the pressure detected with the pressuredetector is equal to or lower than a predetermined value, stops themovement of the tension control roller with the driver.
 5. The imageforming apparatus according to claim 1, wherein the driver, the positiondetector, and the pressure detector are respectively provided incorrespondence with both ends of the tension control roller, and whereinthe controller individually controls driving of the respective drivers.6. The image forming apparatus according to claim 1 further comprisingan ink supply device provided oppositely to an outer peripheral surfaceof the endless belt, wherein the outer peripheral surface of the endlessbelt is used as a conveyance surface for the recording medium, on whichan ink image, formed of ink supplied with the ink supply device, isformed.
 7. The image forming apparatus according to claim 1 furthercomprising a developing device provided oppositely to an outerperipheral surface of the endless belt, wherein the outer peripheralsurface of the endless belt is used as a holding surface for a tonerimage formed of toner supplied with the developing device.
 8. A beltdriving device comprising: a plurality of rollers including a drivingroller; an endless belt stretched around the plurality of rollers; adriver, with one of the plurality of rollers as a tension control rollerto control tension applied to the endless belt, that moves a position ofthe tension control roller in a vertical direction to an axis of anotherroller; a position detector that detects the position of the tensioncontrol roller; a pressure detector that detects pressure applied to thetension control roller with the endless belt; and a controller thatcontrols movement of the tension control roller with the driver,wherein, to set the tension applied to the endless belt as apredetermined value, the controller controls the driver to bring thepressure detected with the pressure detector closer to a target pressureto be added to the tension control roller in the position of the tensioncontrol roller detected with the position detector.
 9. A method ofcontrolling a belt driving device comprising: a plurality of rollersincluding a driving roller; an endless belt stretched around theplurality of rollers; and a driver, with one of the plurality of rollersas a tension control roller to control tension applied to the endlessbelt, that moves a position of the tension control roller in a verticaldirection to an axis of another roller, comprising: detecting theposition of the tension control roller with a position detector;detecting pressure applied from the endless belt to the tension controlroller with the pressure detector; and controlling, to set the tensionapplied to the endless belt as a predetermined value, the driver tobring the pressure detected with the pressure detector closer to atarget pressure to be added to the tension control roller in theposition of the tension control roller detected with the positiondetector, with the controller.